DUST PREVENIION" AND ROAD PEESERVATION, 1915. 33 



concrete. Sand and gravel of the mechanical analyses given in 

 Tables 33 and 34 were used in the concrete, which was mixed in a 

 mechanical mixer and allowed to stand 10 days before the wearing 

 sm-face was apphed. The proportion of the mix, by volume, was 1 

 part cement, 3 parts sand, 7 parts gravel. 



Two types of bituminous concrete were chosen for the wearing 

 course, which was spread so as to compact to a thickness of 2 inches. 

 The northern half of each drive was surfaced with a mixture prepared 

 according to the Topeka specification, while the southern half of 

 each was paved with bituminous concrete prepared according to the 

 District of Columbia specification. The analyses of the bituminous 

 aggregates of these preparations are given in Table 35 and Table 36, 

 respectively. An analysis of the oil-asphalt is given in Table 37. 



The bituminous concrete was prepared in a local plant and was 

 dehvered to the site of the work in trucks at a temperature of 220° F. 

 or more. After being spread and sufficiently cooled the mixture was 

 given a prehminary rolling with a 3-ton tandem roller and was fin- 

 ished with a 12-ton roller. A seal coat of oil-asphalt represented by 

 the analysis given in Table 37 was spread with a squeegee on the 

 areas surfaced with the District of Columbia mixture, and covered 

 with hot stone chips immediately. In the case of the Topeka mixture 

 the seal coat was omitted and the surface was dusted lightly with 

 Portland cement. 



Table 33. — Mechanical analysis of sand used in concrete base. 



Per cent. 



Total retained on 10-mesh sieve 10. 2 



Total retained on 20-mesh sieve 33. 9 



Total retained on 30-me8li sieve 53. 9 



Total retained on 40-mesli sieve 66. 4 



Total retained on 50-mesh. sieve 77. 9 



Total retained on 80-mesli sieve 95. 9 



Total retained on lOO-mesh sieve 96. 5 



Total retained on 200-mesh. sieve 97. 9 



Passing 200-mesh sieve 2. 



Table 34. — Mechanical analysis of gravel used in concrete base. 



Per cent. 



Total retained on l^-inch screen 2. 1 



Total retained on 1-inch screen 5. 5 



Total retained on |-inch screen 18. 1 



Total retained on ^-inch screen 34. 7 



Total retained on \ -inch screen 70. 7 



Total retaine<l on 10-mesh sieve 94. 



Paaaing lO-mesh sieve 6. 



52086°— Bull. 407— IfJ 5 



